1. Field of the Invention
The present invention relates to an X-ray diagnosis apparatus which has a plurality of detection elements arranged in 2-dimensions.
2. Description of the Related Art
Medical diagnosis imaging technology using X-rays, an magnetic resonance imaging (MRI), X-ray computer tomography (CT), or similar technologies have rapidly improved with the advancement of computer technologies. An X-ray film and an Image Intensifier (I.I.) are conventionally used in X-ray diagnosis equipment. When using an I.I., X-rays irradiated through a patient are changed to an optical signal by the I.I., and the optical signal is changed to an electrical signal by a TV camera. The electrical signal is then A/D converted and displayed on a TV monitor as an X-ray image. The I.I. makes it possible to image the patient in real time, which cannot be realized by a method using the X-ray film. Further, since the image is obtained as a digital signal in the I.I. method, various types of image processing can be used to further analyze the image.
In some devices, the traditional I.I. has been replaced with an X-ray flat panel detector (hereinafter flat panel detector). A flat panel detector is disclosed in U.S. Pat. No. 5,818,898, the entire contents of which are incorporated herein by reference. Typically, a flat panel detector has 2-dimensional X-ray detection elements. In comparison with the I.I., the flat panel detector is characterized by improved image quality and stability. In a flat panel detector, an electric charge corresponding to an amount of the X-ray irradiation (hereinafter called as a signal charge) accumulates in the 2-dimensional detection elements. The signal charges accumulated in the detection elements are sequentially output via a TFT (Thin Film Transistor) which has a switching function. The output signal is converted to a signal voltage by a charge/voltage converter which is then converted to a digital signal by an A/D converter. The digital signal is stored in an image memory as an X-ray image, and the X-ray image is displayed on the TV monitor. The digital signal of the X-ray image can be used for various processing to the image or subtraction processing between the images.
In conventional flat panel detectors, spurious electric charges caused by stray capacitances between a gate and a source (hereinafter called as a stray charge) accumulate after the TFT changes from ON state to OFF state. Moreover, it is known that an impedance does not become a large value immediately after the impedance between the source and a drain of the TFT changes OFF. Consequently, the stray charge can leak out to the output wire through a gap between the source and the drain, and can thus be inadvertently added to the signal charge output via the TFT. Because the signal charges accumulated in each the detection elements are read according to a similar principle as a TV scan, an amount of the added stray charges (hereinafter called as a total stray charge) increases according to the line number corresponding to the line where the signal charge is read out. That is, the total stray charge is different in each line and is added to the signal charge which is used for making the X-ray image. These accumulating stray charges cause a phenomenon called as shading), whereby each line is corrupted with different brightnesses, making the image quality low.